Hydrogels represent a desirable class of materials for the manufacture of various biomedical devices, including ophthalmic devices such as contact lenses. A hydrogel is a hydrated cross-linked polymeric system that contains water in an equilibrium state. Hydrogel lenses offer desirable biocompatibility and comfort. Silicone hydrogels are a known class of hydrogels and are characterized by the inclusion of a silicone-containing material. Typically, a silicone-containing monomer is copolymerized by free radical polymerization with a hydrophilic monomer, with either the silicone-containing monomer or the hydrophilic monomer functioning as a crosslinking agent (a crosslinker being defined as a monomer having multiple polymerizable functionalities) or a separate crosslinker may be employed. An advantage of silicone hydrogels over non-silicone hydrogels is that the silicone hydrogels typically have higher oxygen permeability due to the inclusion of the silicone-containing monomer.
Various polysiloxane-based prepolymers with urea or urethane linkages have been disclosed as potential silicone-containing monomers for silicone hydrogels. These various prepolymers may differ in their methods of preparation and in their physical characteristics/properties, and thus may exhibit divergent behavior when combined with other monomers for hydrogel copolymers.
One class of urethane- or urea-containing polysiloxane prepolymers involves polysiloxane-diol or polysiloxane-diamine endcapped with an ethylenically unsaturated monomer having an isocyanate group, such as isocyanatoethyl methacrylate (IEM). For example, by reacting IEM with hydroxy-terminated polydimethylsiloxane, a prepolymer is formed. In general, this class of prepolymer exhibits similar compatibility with hydrophilic monomers as corresponding prepolymers without the urethane linkage, especially for higher molecular weight prepolymers. Also, in general, these prepolymers are liquid at room temperature. Examples of such prepolymers are found in U.S. Pat. No. 4,605,712 (Mueller et al.).
A second class of urethane-containing polysiloxane prepolymers employs a diisocyanate to create urethane linkages. In general, these prepolymers are prepared by reacting 2 moles of diisocyanate with a hydroxy-terminated polydimethylsiloxane, followed by end capping with 2-hydroxyethyl methacrylate (HEMA). This class exhibits slight improvement in compatibility with hydrophilic monomers such as N,N-dimethylacrylamide (DMA), depending on the molecular weight of polysiloxane. Also, in general, this class of prepolymers is liquid at room temperature. Examples of such prepolymers are found in U.S. Pat. No. 4,136,250 (Mueller et al.)
U.S. Pat. No. 5,034,461 (Lai et al.) discloses various polysiloxane-containing urethane or urea prepolymers. Generally, these prepolymers are derived from a short chain diol, a hydroxy-terminated polydimethylsiloxane and a diisocyanate, such that the structures resemble a segmented polyurethane elastomer; these prepolymers are endcapped with polymerizable ethylenically unsaturated radicals, such as HEMA reacted with isocyanate. These prepolymers may be copolymerized with a hydrophilic comonomer to form a silicone hydrogel copolymer that is useful as a contact lens material or other biomedical device applications. The preferred prepolymers of U.S. Pat. No. 5,034,461 are composed of soft polysiloxane segments (represented by A in the patent formulae) and strong hard segments (represented by *D*G*D* in the patent formulae), and are endcapped with polymerizable ethylenically unsaturated radicals.
The polysiloxane-containing prepolymers of this invention comprise soft and strong hard segments as in U.S. Pat. No. 5,034,461. However, the prepolymers of this invention further comprise relatively weaker hard segments, in addition to the soft and strong hard segments. It has been found that this leads to several advantages. First, the present prepolymers tend to have a lower viscosity at room temperature, making them easier to process during synthesis and in casting biomedical devices as compared to prepolymers disclosed in U.S. Pat. No. 5,034,461. Second, this arrangement permits forming a prepolymer of higher silicone content, thereby permitting the formation of copolymers with higher oxygen permeabilities while maintaining good compatibility with hydrophilic monomer and forming clear hydrogels, as compared to prepolymers disclosed in U.S. Pat. Nos. 4,136,250 and 4,605,712. Third, because of the arrangements of the various segments, copolymers offering the higher oxygen permeabilities can be obtained without high modulus.